Oils are used as carbon sources in antibiotic fermentations both for economic reasons and because higher antibiotic titres often result. The model organism, Streptomyces lividans, grew to a higher cell density and produced the antibiotic, actinorhodin, faster with a combination of glucose and the triacylglycerol triolein than with either substrate alone. When triolein was sole substrate, beta-oxidation rates were maximal and there was no evidence of de novo fatty acid biosynthesis. With glucose as the sole carbon source, fatty acid biosynthesis took place and there was no beta-oxidation. With the mixed substrates, there was neither beta-oxidation nor fatty acid biosynthesis, indicating that glucose provided all the intermediates down to the level of acetate and that the triolein provided the majority of the straight-chain fatty acyl groups in the lipids. Thus, the cells use both substrates simultaneously, evidently by mutually exclusive mechanisms, thereby achieving increased growth and productivity and explaining why such mixed nutrients are beneficial in Streptomyces fermentations.